BH growth via mergers is expected to have a dominant role in the formation of SMBHs (with respect to gas accretion), especially for \(<10^{4-5} M_\odot \)  BHs at high redshift
(e.g., [1]}, [2]}, [3]}).
However, BH growth via mergers only can not explain the formation of \(z>7\)  SMBHs, and high accretion rates are crucial in the growth of high-redshift SMBHs, almost independently of the seed mass (e.g., [4]}, [5]}, [6]}).
In addition, the asymmetric emission of GWs in the merger process may lead to the ejection/displacement of the newly formed BH from the galaxy centre (gravitational recoil). Thus the BH may be removed from the reservoirs of dense gas from which it accretes (e.g., [7]}). The high velocities imparted to the recoiling BH (often called kick velocities) may have an impact on the growth of SMBHs, especially at high redshift (\(z>15\) ; e.g., [5]}, [9]}).
